Friday, May 13, 2016

Probe Terminology (from Teledyne LeCroy's Blog)

As oscilloscope users, we know that the probe is a critical element in
getting signals from the device under test into the instrument. The
ideal probe would have perfectly flat magnitude response and perfectly
linear phase response across its entire frequency range. Unfortunately,
that probe, though striven for by all oscilloscope manufacturers, does
not exist.

What does exist is a good deal of confusion about
what can be done to coax probes to behave more like that ideal probe. We
often hear terms such as "calibration," "correction," "compensation,"
and "de-embedding" tossed around, often interchangeably. All of them do
involve how the measurement system accounts for the probe's effect on
the signal under test. But let's take a look at them and see how they
actually differ in practice.

One possible approach is to employ a calibrated, precision instrument --
usually a vector network analyzer (VNA) -- to measure the probe's
actual magnitude and phase response. With those measurements in hand,
one might build a correction filter that removes the undesired effects of the probe's frequency response characteristics from the acquired signal.

Using a precision instrument to measure a probe's performance and making
some adjustment (the correction) to its output to nudge it toward ideal
performance is, by definition, a calibration. Why? Because it
involves measurements made to traceable standards. The correction is but
a part of this overall calibration process.

Then there's compensation, which should be familiar to anyone
who's used a common bench oscilloscope and a passive probe. The probes
usually come with a little plastic screwdriver. When you connect the
probe to the oscilloscope, you use the little screwdriver to turn a
trimmer capacitor on the probe's plug end. In doing so, you adjust the
probe's output impedance to match the input impedance of the
preamplifier on the input channel you've plugged it into.

With high-bandwidth oscilloscopes, the compensation process is a little
different in that the probe correction and scope-channel correction are
convolved together. As a result, the entire measurement system behaves
in a very controlled fashion across the instrument's full rated
bandwidth and even beyond.

These days, one will come across the concept of probe de-embedding.
which involves accounting for reflections from the probe tip along the
transmission line that it's connected to. Using models of the probe
tip's loading impedance, the impedance profiles of the transmission
line, and components in the circuit, one may account for reflections
from components that travel back to the probe tip and affect
measurements. Usually, the probe loading is enough that reflections from
the probe are minimal, making the need for probe de-embedding a
relatively rare one.